Sorry for the short reply but I'm writing a paper at the moment.
These phones should have the sensor for when you put the phone up to your face, the change in light turns the screen off so that you don't press anything with your cheek until you pull it away. I don't know if its possible to turn this option off, but you might want to check the settings to make sure it is in fact on and working. My S3 has the sensor near the earpiece, and I know a few generations of phones including older iphones also have this feature so I wouldn't see why yours don't. My sister has the Galaxy Nexus and I believe hers also has the feature.
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